While we usually refer to Exodus 20 as the Ten Commandments, and many versions of the Bible add the heading "The Ten Commandments", the phrase is not actually used there in the text of the Bible. (It does, however, appear later on, in Exodus 34:28, Deuteronomy 4:13 and Deuteronomy 10:4). Also, while we know that the original ten commandments were written in stone, the commandments in Exodus 20 are not written in stone. This happens in Exodus 24:12.
How do we know that these were the Ten Commandments given that the "Ten Commandments" were not referenced as such until later on?
This question (and its accepted answer) nicely addresses the differences in interpretations of Exodus 20 as the Ten Commandments, but how can we be sure that the Ten Commandments are what's listed in Exodus 20?
Furthermore, I once heard it suggested that what's written in Exodus 20 - the "you shall not"s - are promises, not commandments. This makes sense, especially as there follow some instructions that definitely are commandments (Exodus 20:23 onwards) - "do not"s instead of "you shall not"s.
Why the difference in language ("you shall not / do not")?